A cellular telephone system is, as the name implies, built up by cells each covering a certain smaller area of a greater territory assigned to an operator giving mobile telecommunication service to its subscribers. An example of such a cell configuration is shown in FIG. 1. Each cell has a base station communicating with mobile stations occupying the area of the cell. A certain number of radio frequencies are allocated to the operator and a task for him is to optimise his resources in serving his subscribers. As the number of frequencies is limited they must be used efficiently. Depending on the services one or several frequencies are used in a particular cell. None of these frequencies may be used in neighbouring cells because of interference. The reuse distance, that is the distance to another cell area where the same frequency can be used again without interference, is at least the cross-distance of the neighbouring cell, assuming that the radio coverage area of a base station is its own cell area and partly into the neighbouring ones but not beyond that.
The sizes of the cells depend on the radio propagation distance and the number of users in the cells. In urban environments, where the number of users can be quite high, the cells tend to get small. Especially with these small cells and also with a complicated topography, big houses, indoor and underground users, etc, the ideal picture of the cell configuration as shown in FIG. 1 is certainly not true. A more likely situation is depicted in FIG. 2. As can be seen in the figure the coverage of a base station is not just the area surrounding it. Spots of coverage could be found even far away from the base station. On the other hand there are places close to a base station, which are shadowed for example by a building. In that case transceivers situated further away have to be used for the system to work even in those shadowed places.
Such a telephone system is configured, to start with, by making a rough estimation of coverage and placing out the base stations. Thereafter the system is refined by using test mobiles moving around in the area and reporting back measurement results to the system controller. Based on these results the system may be reconfigured. Also the normal users of the system report measurement results. In fact, the whole thing is an ongoing process taking accounts of the dynamic behaviour of the users and the changes in the environment. The goal is to have an optimised system at all times.
Another object of the measurements is for the mobile to know which base station to turn to when using the system. Depending on where the mobile is located, a list of suitable frequencies is sent out to the mobile for it to listen to and choose from when it wants to contact the system and making handovers as the mobile, for example, moves into other coverage areas.
In a Time Division Multiple Access, TDMA, cellular system like the GSM-system, the information is physically sent out in the form of bursts in short time slots. In GSM eight time slots form a TDMA-frame, which continuously repeats itself during transmission. The type of message sent out on these physical channels categorises the channels as Logical Channels. There are two basic types of logical channels, Control Channels, CCH, for control purposes and Traffic Channels, TCH, which carry the actual payload. One of the standardised control channels is the Broadcasting Control Channel, BCCH, which broadcasts information about the base stations, e.g. its transmitting frequency and the so called Base Station Identity Code, BSIC. The BSIC is composed of just a few bits and may discriminate between the closest neighbouring base stations. The BCCH is sent out at regular intervals in a certain time slot, normally the first (i.e time slot number zero) and with no power reduction. The mobiles are programmed to listen to the BCCHs sent out by neighbouring base stations and report back to the system measurement results concerning signal strength and quality concerning the identified BCCHs. The purpose of the measurements is, as mentioned above, to help refining the system configuration and to facilitate handovers.